This invention is a method and instrument for the detection of fissile material and certain additional nuclear materials (NM) through non-destructive interrogation using gamma rays.
There is an ongoing problem concerning the detection of contraband fissionable material and other nuclear materials (NM). The threat from these materials could be substantially reduced by the development of an effective, compact, transportable inspection system that could be used at critical locations such as border crossings and sensitive nuclear facilities.
Other technologies for the detection of clandestine fissile materials have been suggested. These include neutron-induced fission which involves the stimulated emission of neutrons by primary neutrons. The disadvantage with this suggested technique is the difficulty of differentiating the primary and secondary neutrons that is required to establish an unambiguous signal for the clandestine material. The concept of interrogation by photofission has also been suggested, but in these earlier approaches the implementation is based on a continuum bremmstrahlung spectrum generated by an electron linac. With this technique, most of the gamma-ray dose comes from energies below the photofission or photo-neutron emission thresholds. So, such a device involves a high radiation dose to the object being interrogated and to the environment with corresponding low contraband material detection efficiency associated with each gamma-ray generated. If the linac is operated at higher voltages to improve the yield of effective gamma-rays, it is easy to exceed the photo-neutron threshold for benign materials; thereby leading to a false positive signal. The subject invention does not suffer these limitations. With the subject invention protons are accelerated by a compact low-energy accelerator, Epxe2x89xa64 MeV. They impinge on a thick, stopping, stable target consisting of a pure fluorine compound, such as CaF2, MgF2, AlF3, SF6 etc., bonded to a thermally conducting backing or in gaseous form. An intense, nearly monoenergetic, high-energy gamma-ray source (6-7 MeV xcex3""s) is established generating gamma rays via the 19F(p,xcex1xcex3)16O reaction. At the generated proton energies, no primary neutrons are emitted from the cited targets. For 6-7 MeV gamma ray energies, both photofission and photo-neutron reactions are feasible for actinide materials. The photofission leads to the production of prompt-fission neutrons (nu-bar≈2-3 per fission). Furthermore, other NM such as deuterium, lithium-6, and beryllium can also be detected through photo-neutron reactions which are energetically feasible at these gamma ray energies. Most benign materials will not respond to these particular gamma rays through the emission of neutrons. Therefore, observation of neutron events by a detector with a high degree of gamma-ray discrimination provides an unambiguous signal of these contraband materials.
Thus, the object of this invention is to provide an apparatus and method for the detection of fissionable material through non-destructive interrogation using gamma rays. This apparatus would also be capable of distinguishing the isotope uranium-235 (U-235, which is fissile) from uranium 238 (U-238, which is fissionable).
Additional advantages, objects and novel features of the invention will become apparent to those skilled in the art upon examination of the following and by practice of the invention.
To achieve the foregoing and other advantages, this invention is an apparatus and method for detecting fissionable material and other nuclear materials (NM) through non-destructive interrogation using gamma-rays. The gamma-rays are generated by directing accelerated protons at a Fluorine-compound target. The interaction of the protons with the target produces nearly monoenergetic, high energy gamma rays. These gamma rays impinge upon the item being interrogated. If the item of interest contains fissile or fissionable material or other nuclear material, neutrons are produced by photo-fission and/or photo-neutron reactions. Neutron detectors are placed at specific locations in relation to the item being interrogated. If the detectors register the presence of neutrons, that is a strong indication that the item contains either fissionable material or other nuclear material.